The invention relates to discharge lamps and particularly to self-ballasted lamps having a starting electrode. The usual construction for self-ballasted lamps having a starting electrode is an incandescent filament in series with an arc tube. It will be understood that the terms arc tube, discharge tube and burner are used interchangeably herein. Self ballasted lamps designed to operate on 120V power supply having a starting electrode disposed at at least one end thereof. The starting electrode is internal to the arc tube and is in series with a bi-metallic switch. The bi-metallic switch which is external to the arc tube is normally closed and opens upon rise in temperature such as is normally encountered by operation. A problem with such bi-metallic switches is that the reliability of the switches often determines the life of the lamp. Since such lamps are frequently used for applications where the service life is particularly important it is highly desirable to provide lamps having a greater service life. It will be understood that in those lamps utilizing the bi-metallic or thermal switch that if the switch does not close at all the main electrode is never heated by the heater coil. The lamps customarily do not have sufficient potential across the burner to initiate electron emission without benefit of the heating of the electrode and accordingly starting will not occur. In addition if the switch never opens, the mercury burner is continuously short-circuited and the ballast coil sustains approximately double design operating voltage until it burns out. If the burner does start and the bi-metallic switch does not stay open upon reaching normal steady state operating temperature, blinking will occur. The arc will strike upon opening of the switch and will then be shorted out as the switch recloses. Cyclical operation in this manner will continue until the ballast coil burns out or power is removed.
It is known in discharge lamps to use voltage multiplier circuits to increase the potential between electrodes. Typically they operate continuously during the operation of the lamp. The continuous operation of the doubler circuit is undesirable because it shortens the life of the starting electrode.
Frequently discharge lamps have required the use of external ballast to limit current to the arc tube or to limit the rate of change of current flow to the arc tube. The use of an external ballast complicates the installation of lamps of the discharge type and accordingly it is particularly desirable to have lamps which are self-ballasted.
Accordingly it is a primary object of the invention to provide a discharge lamp having a high reliability and a long life.
Another object of the invention is to provide a discharge lamp which eliminates the requirement for a thermal switch.
Still another object of the invention is to provide a discharge lamp which is self-ballasted and in which the ballast provides at least a portion of the light output of the lamp.